1. Field of the Invention
This invention relates to a magnetic reproducing apparatus and a reproducing amplifier circuit to be applied to this apparatus. In particular this invention relates to a reproducing amplifier circuit for a MR head that is adopted to a Voltage-Bias Voltage-Sense type reproducing system.
2. Description of the Related Art
In a recording and/or reproducing apparatus such as a HDD (Hard Disk Drive) apparatus, preferably a MR (Magneto-Resistive) head is used in order to reproduce data recorded on a magnetic recording hard disc. In this MR head, the resistance of the MR head changes according to the-change of a magnetic field generated based on recorded data on the magnetic recording hard disc.
As one of reproducing system for such MR head, a Voltage-Bias Voltage-Sense type reproducing system is introduced, wherein the change of the resistance of the MR head corresponding to the recorded data is derived as an output voltage by applying a predetermined bias voltage to the MR head.
FIG. 1 shows a conventional circuit diagram for a MR head, wherein a feedback loop is formed in a voltage-current converting circuit Gm2 in order to supply a reference voltage V1 to a MR head MRH. Namely, a constant voltage supply amplifier circuit AMP1 is connected to one node voltage terminal ND1 of the MR head MRH and the other node voltage terminal ND2 is connected to ground GND. The constant voltage supply amplifier circuit AMP1 includes the above mentioned voltage-current converting circuit Gm2. One input terminal (+) of the voltage-current converting circuit Gm2 is connected to the reference voltage V1, and the other input terminal (xe2x88x92) thereof is connected to the output side for feedback operation, namely to an output connected to the node voltage terminal ND1 of the MR head MRH. A transistor Q4 is connected to an output of the voltage-current converting circuit Gm2, beside a capacitor C1, in order to take out a detecting signal from the MR head MRH. Thereby, the voltage of the node voltage terminal ND1 of the MR head MRH is fixed to the reference voltage V1, and the change of the resistance of the MR head MRH is taken out as a collector current of the transistor Q4. The current flowing through the MR head MRH is called a bias current IB and is expressed with the next expression:
IB=V1/Rmr
In the above expression, the Rmr shows a resistance of the MR head MRH. The collector current of the transistor Q4 corresponding to the change of the resistance of the MR head MRH flows through a resistor R9, and the DC (Direct Current) component included in the signal components generated across the resistor R9 is cut by a capacitor C5, and thereby the recorded data is reproduced by amplifying alternate current components in the signal components by a differential amplifier A2. In FIG. 1, a reference bias voltage V3 is supplied to a mid-point of series-connected resisters R10 and R11 connected between two input terminals of the differential amplifier A2 for deciding an operating bias point of the differential amplifier A2. Further a capacitor C4 is connected between a power source Vcc and one of the input terminals of the differential amplifier A2.
However, in the conventional reproducing amplifier circuit of the voltage bias system, one of terminals of the MR head MRH is connected to ground GND, and thereby when static electricity is supplied to the MR head MRH due to external noises or other reason, a rush current corresponding to the static electricity divided by the resistance Rmr of the MR head MRH resultantly flows. Normally, the resistance Rmr of the MR head shows around 30xcexa9 to 80xcexa9, and when a large amount of rush current flows due to the static electricity, the problem of the MR head being destroyed by the rush current will possibly occur.
In addition, the signal taken out as the collector current of the transistor Q4 is converted to the signal voltage by the resistor R9, and this signal voltage is supplied to the non-inverting input terminal of the differential amplifier A2. On the other hand, the inverting input terminal of the differential amplifier A2 is merely biased by the reference bias voltage V3. Namely, there is formed a circuit for converting signal components obtained as a single-type output through the resistor R9 and the capacitor C5 into a differential type output.
In such a construction, the input section of the differential amplifier A2 is not completely symmetric and, accordingly, ripple components of the power source Vcc are supplied as differential components through the pair of input terminals to the differential amplifier A2. On this account, the ripple components in the power source Vcc are not completely removed, and this causes deterioration of high frequency characteristics in the reproducing circuit for the MR head installed in the HDD apparatus, for example.
The primary objective of the present invention are to prevent the destruction of the MR head by a large amount of rush current when static electricity is applied thereto and to present a reproducing amplifier for a MR head capable of supplying an output signal without affecting the high frequency property in spite of ripple components included in the power source.
According to the present invention, in order to achieve the above objectives, a novel reproducing amplifier for a MR head is proposed, wherein a MR head is interposed between a pair of current source circuits and a feedback amplifier is provided for controlling the current of the pair of current source circuits so that the terminal voltage of the MR head becomes a predetermined value.
According to this construction for a reproducing amplifier circuit of the MR head, the MR head is connected between the two current sources, so that when static electricity is applied to the MR head, the voltage difference due to the applied static electricity does not appear at the terminals of the MR head, and thereby the MR head is protected from destruction by a large amount of current due to the applied static electricity.
In addition, when ripple components are included in the power source by connecting a differential amplifier to the MR head in order to derive the output signal, the terminal current of the MR head due to the ripple components is changed in-phase, so that the ripple components do not appear in the output signal and thereby the high frequency characteristic of the output signal is not deteriorated by the ripple components.
In a preferred construction of one example of the present invention, the above-mentioned feedback amplifier has a predetermined cut-off frequency, and it keeps the terminal voltage of the MR head at a value which remains constant to a signal having a frequency band below the cut-off frequency. On the contrary, the feedback amplifier changes the terminal voltage of the MR head to a signal having a frequency band above the predetermined cut-off frequency through a change of resistance of the MR head by applying a constant current through the pair of current source circuits.
According to this preferred construction, if the cutoff frequency of the feedback amplifier is set to a lower frequency than the frequency band where resistance change due to noises, the feedback amplifier keeps the terminal voltage of the MR head constant to the change of the resistance in the frequency lower than the cut-off frequency, and thereby the reproducing operation is not done.
On the contrary, the feedback amplifier does not operate to keep the terminal voltage of the MR head constant to the change of the resistance in the frequency higher than the cut-off frequency, so that output signal is generated as the terminal voltage of the MR head due to the recorded data, and thereby the reproducing operation is done. The change of the terminal voltage of the MR head is derived as the reproduced output signal.
In a more preferred construction of this invention, a two series-connected resisters having a resistance larger than that of the MR head is connected between two terminals of the MR head and the junction point of the two resistors is connected to ground.
According to the above construction, even when error components are produced in a DC current from the two current sources due to manufacturing dispersion, a mid-point voltage of the MR head is determined by the product of the error amount of the two current source circuits and the resistance of the two resistors connected in parallel, and thereby the mid-point voltage is always kept at the ground potential.